Toe initiator having an associated object catching seat

ABSTRACT

A technique includes installing a tubing string having a toe initiator valve in a wellbore; and opening the toe initiator valve to establish fluid communication between an interior of the string and a region outside of the string. The technique includes using the fluid communication established by the open toe initiator valve to communicate a degradable object downhole to land in a seat of the string; and performing a downhole operation using a fluid barrier resulting from the landed object.

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/110188 filed on Jan. 30, 2015, incorporated by reference in its entirety.

BACKGROUND

For purposes of preparing a well for the production of oil or gas, at least one perforating gun may be deployed into the well via a conveyance mechanism, such as a wireline, slickline or a coiled tubing string. The shaped charges of the perforating gun(s) are fired when the gun(s) are appropriately positioned to perforate a casing of the well and form perforating tunnels into the surrounding formation. Additional operations may be performed in the well to increase the well's permeability, such as well stimulation operations and operations that involve hydraulic fracturing. The above-described perforating and stimulation operations may be performed in multiple stages of the well.

The above-described operations may be performed by actuating one or more downhole tools (perforating guns, sleeve valves, and so forth) and by forming one or more fluid-diverting fluid barriers downhole in the well.

SUMMARY

The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In accordance with an example implementation, a technique includes installing a tubing string having a toe initiator valve in a wellbore; and opening the toe initiator valve to establish fluid communication between an interior of the string and a region outside of the string. The technique includes using the fluid communication established by the open toe initiator valve to communicate a degradable object downhole to land in a seat of the string; and performing a downhole operation using a fluid barrier resulting from the landed object.

In accordance with another example implementation, a technique that is usable with a well includes opening a toe initiator valve of a tubing string; pumping a degradable object downhole to land in a seat associated with the toe initiator valve; and performing pressure testing of the string using a fluid barrier created due to the landing of the degradable object in the seat.

In accordance with another example implementation, an apparatus that is usable with a well includes a string, a toe initiator valve and a seat. The string is associated with a plurality of stages, and a first stage of the plurality of stages is closest to a toe end of the string. The toe initiator valve is disposed in the string near the toe end of the string and includes a radial port to communicate fluid with a region outside of the string. The seat disposed in the string between the toe end of the string and the first stage to receive an object to form a fluid barrier between the port of the toe initiator valve and the first stage.

In accordance with yet another example implementation, an apparatus includes a toe initiator valve to open in response to a pressure of a tubing string in which the valve is installed exceeding a pressure threshold. The toe initiator valve includes a coupler to connect the toe initiator valve to a string; at least one radial port to communicate fluid in response to the toe initiator valve being open; and a seat disposed between the coupler and the port to receive an untethered object communicated through the string.

Advantages and other features will become apparent from the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, 1E and IF are schematic diagrams of a well illustrating operations performed in the well using a toe initiator valve having an associated object catching seat according to an example implementation.

FIG. 2A is a partial cross-sectional view of the toe initiator valve in a closed state according to an example implementation.

FIG. 2B is a partial cross-sectional view of the toe initiator valve in an open state according to an example implementation.

FIG. 3 is a flow diagram depicting a technique to use an object catching seat of a toe initiator valve to perform a downhole operation according to an example implementation.

FIG. 4 is a flow diagram depicting a technique to use an object catching seat of a toe initiator valve to perform pressure testing and well stimulation operations in a well according to an example implementation.

FIG. 5 is a partial cross-sectional view illustrating an assembly containing an object catching seat and a toe initiator valve according to an example implementation.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth but implementations may be practiced without these specific details. Well-known circuits, structures and techniques have not been shown in detail to avoid obscuring an understanding of this description. “An implementation,” “example implementation,” “various implementations” and the like indicate implementation(s) so described may include particular features, structures, or characteristics, but not every implementation necessarily includes the particular features, structures, or characteristics. Some implementations may have some, all, or none of the features described for other implementations. “First”, “second”, “third” and the like describe a common object and indicate different instances of like objects are being referred to. Such adjectives do not imply objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. “Coupled” and “connected” and their derivatives are not synonyms. “Connected” may indicate elements are in direct physical or electrical contact with each other and “coupled” may indicate elements co-operate or interact with each other, but they may or may not be in direct physical or electrical contact. Also, while similar or same numbers may be used to designate same or similar parts in different figures, doing so does not mean all figures including similar or same numbers constitute a single or same implementation. Although terms of directional or orientation, such as “up,” “down,” “upper,” “lower,” “uphole,” “downhole,” and the like, may be used herein for purposes of simplifying the discussion of certain implementations, it is understood that these orientations and directions may not be used in accordance with further example implementations.

In accordance with example implementations, for purposes of completing a wellbore, a tubing string (a casing string, for example) may be run into and installed in the wellbore for purposes of lining and supporting the wellbore. At its distal, or far, end, the tubing string may include a valve called a “toe initiator valve.” The toe initiator valve has an open central passageway and radial ports, and the toe initiator valve may be remotely operated to control fluid communication through its radial ports. In general, the toe initiator valve has two states: a closed state in which the toe initiator valve blocks fluid communication through the radial ports; and a closed state in which the toe initiator valve allows fluid to be communicated through the radial ports.

When the tubing string is initially run into and installed in the wellbore, the toe initiator valve is in its closed state, and the interior of the tubing string may be a closed space, which means a fluid flow cannot be communicated through the tubing string. A fluid flow inside the tubing string may be used for various purposes, such as communicating stimulation fluid downhole, hydraulic fracturing, pumping tools downhole, and so forth.

For purposes of establishing a fluid flow inside the tubing string, the toe initiator valve is opened, which permits a fluid flow from the interior of the tubing string to a region outside of the tubing string. One way for the toe initiator valve to be opened is through the use of a well intervention operation in which a tool is run inside the tubing string to engage the valve. For example, the toe initiator valve may be constructed to be mechanically opened by a shifting tool. In this manner, a shifting tool may be run downhole inside the central passageway of the tubing string for purposes of engaging a sleeve of the valve to shift the sleeve to open the radial ports.

The toe initiator valve may be constructed to be opened without a well intervention. For example, the toe initiator valve may be constructed to shift a sleeve to open communication through the radial ports in response to the pressure inside the interior of the tubing string exceeding a certain activation pressure threshold. After the installation of the tubing string (after cementing of the string in place for implementations in which the tubing string is a casing, for example), a pressure test may be performed on the string (e.g., a casing pressure test may be performed). One way to allow such a pressure test to be conducted is to construct the toe initiator valve to open with pressure and have a pressure opening threshold that exceeds the pressures involved in pressure testing the tubing string. Thus, the toe initiator valve remains closed (i.e., remains in its initial state) during the pressure testing and may be subsequently opened to initiate a fluid flow by pressurizing the interior of the tubing string with a pressure higher than the pressures associated with the pressure testing.

In accordance with example systems and techniques that are discussed herein, a tubing string installed in a well has a toe initiator valve, which is constructed to open at a tubing pressure that is less than the maximum pressure used to pressure test the tubing string. Thus, the closed state of the toe initiator valve may not be used to seal off the interior space of the tubing string for pressure testing. Instead, the toe initiator valve has an associated object catching seat, which is constructed to receive an object to form a temporary fluid barrier to seal off the toe end of the tubing string. More specifically, in accordance with example implementation, toe initiator valve may be opened (by pressurizing the tubing string, for example) before pressure testing. To isolate the toe initiator's port(s) to allow the pressure testing to occur, the toe initiator valve has an associated object catching seat, which is used to catch an object to form a temporary fluid obstruction, or fluid barrier, uphole of the valve. The object is at least partially formed from a degradable material, which means that the fluid barrier formed from the object disappears with the passage of time. In this manner, the degradable material is constructed to remain intact and structurally sound for a certain period of time for purposes of allowing pressures uphole of the object to build up to complete the pressure testing and isolate the open port(s) of the toe initiator valve from the pressure testing. However, eventually, the degradable material degrades to an extent that removes the fluid barrier formed by the object. After this occurs, flow may be re-established inside the tubing string using the open port(s) of the toe initiator valve. Moreover, as further described herein, another object (degradable object, for example) may be deployed to land in the object catching seat to form another temporary fluid barrier for other purposes, such as, for example, diverting fluid for a subsequent well stimulation operation in the stage above the toe initiator valve.

In accordance with example implementations, the object catching seat is constructed to receive, or catch, an untethered object that is pumped downhole through the central passageway of the tubing string. In this context, an “untethered object” refers to an object that is communicated downhole through the passage of a tubing string along at least part of its path without the use of a conveyance line (a slickline, a wireline, a coiled tubing string, and so forth). As examples, the untethered object may be a ball (or sphere), a dart or a bar. Regardless of its particular form, the untethered object travels through the passageway of the tubing string to land in the object catching seat that is disposed above the toe initiator valve to form a corresponding fluid obstruction or fluid barrier.

As a more specific example, FIG. 1A depicts a well 100 in accordance with example implementations. The well 100 includes a laterally extending wellbore 120, which traverses one or more hydrocarbon-bearing formations. As depicted in FIG. 1A, the wellbore 120 is lined and supported by a tubing string 130. The tubing string 130 may be cemented to the wellbore 120 (i.e., the tubing string 130 is a casing); or the tubing string 130 may be secured to the surrounding formation(s) by packers (such wellbores typically are referred to as “open hole wellbores”). For the specific example of FIG. 1A, the tubing string 130 is a casing that has been run into the wellbore 120, and a cementing operation has been formed to place cement 140 in the annular region between the exterior of the casing and the wall of the wellbore 120.

It is noted that although FIG. 1A and other figures that are discussed herein depict a laterally extending wellbore, the techniques and systems that are disclosed herein may likewise be applied to vertically extending wellbores. Moreover, in accordance with example implementations, the well 100 may contain multiple wellbores, which contain tubing strings that are similar to the illustrated tubing string 130 of FIG. 1A. The well 100 may be a subsea well or may be a terrestrial well, depending on the particular implementation. Additionally, the well 100 may be an injection well or may be a production well. Thus, many implementations are contemplated, which are within the scope of the appended claims.

As depicted in FIG. 1A, the tubing string 130 extends from a heel end 141 of a lateral segment 121 of the wellbore 120 to a toe end 143 of the segment 121. The lateral segment 121 may be associated with multiple stages, which are isolated and stimulated separately, as further discussed herein.

The toe end 143 of the tubing string 130 includes a casing shoe 161 and a toe initiator valve 150. As depicted in FIG. 1A, the toe initiator valve 150 includes one or multiple radial ports 160. In its open state, the toe initiator valve 150 allows fluid communication through the radial port(s) 160 to thereby allow fluid communication between the interior space of the tubing string 130 and the region outside of the tubing string 130. Such an open state may be used for purposes of establishing a fluid flow inside the tubing string 130 after the tubing string 130 has been run into and installed in the wellbore 120. In this regard, before the toe initiator valve 150 is opened, the interior of the tubing string 130 may be an otherwise closed system.

FIG. 1A also depicts an object catching seat 151 that is associated with the tubing string 130. As depicted in FIG. 1A, the object catching seat 151 is disposed uphole of the toe initiator valve 150. The object catching seat 151, in accordance with example implementations, is a restriction or narrowing of the central passageway of the tubing string 130. In accordance with example implementations, the object catching seat 151 is constructed to catch an untethered object that is deployed inside the central passageway of the tubing string 130. As described further herein, depending on the particular implementation, the object catching seat 151 may be a component of the tubing string 130 separate from the toe initiator valve 150 or may be part of the valve 150. Regardless of its form, the object catching seat 151 may be disposed in the string 130 between the port(s) 160 of the toe initiator valve 150 and the stage (of the plurality of stages) of the wellbore 120, called the “first stage” herein, which is closest to the toe end 143 of the wellbore 120.

In accordance with example implementations, the toe initiator valve 150 may be initially closed when run into the wellbore 120 with the tubing string 130, and the toe initiator valve 150 may be constructed to be remotely operated by pressurizing the interior of the tubing string 130 to a pressure that causes the valve 150 to open the valve's port(s) 160 without the use of a well intervention. More specifically, referring to FIG. 1B, in accordance with example implementations, for purposes of delivering the untethered object downhole, fluid communication is first established inside the central passageway of the tubing string 130. In this manner, as shown in FIG. 1B, fluid pressure inside the central passageway of the tubing string 130 may be increased to a pressure threshold level that causes the tubing initiator valve 150 to open, thereby permitting a flow 170 that may extend through the radial ports 160 into the region (as depicted at reference numeral 172) exterior to the string 130.

As an example, in accordance with some implementations, the toe initiator valve 150 may be constructed to transition from its closed state to its open state in response to the tubing string pressure at the valve 150 exceeding 10,000 pounds per square inch (psi), which may be less than the maximum pressure (12,000 psi, for example) used for pressure testing the tubing string 130.

Using the flow 170, an untethered object, such as an activation sphere, or ball 174, may be pumped downhole and land in the object catching seat 151, as depicted in FIG. 1C. For this state of the well 100, the landing of the ball 174 in the seat 151 creates a downhole fluid obstruction, or fluid barrier, uphole of the toe initiator valve 150. This fluid barrier, in turn, may be used to seal off the bottom end of the tubing string 130 to isolate the toe initiator valve 150 and allow a pressure test to be performed on the tubing string 130 without change the state of the valve 150.

The ball 174, in accordance with example implementations, contains one or multiple degradable material. Depending on the particular implementation, the ball 174 may contain one or multiple non-degradable materials, may be solid, may be hollow, may be formed from interlocking pieces, and so forth. Regardless of its particular form, the ball 174 is constructed to initially provide a structural integrity that permits a fluid column uphole of the ball 174 to be pressurized (pressurized to perform pressure testing of the tubing string 130, for example) and subsequently degrade in a relatively short interval of time (one week, a few weeks, a month, less than six months, and so forth), as compared to the degradation rates of other equipment in the well (the tubing string 130, for example). The degradation of the ball 174 allows the ball to leave or exit the object catching seat 151 after this relatively short interval of time. For example, the object catching seat 150 may have a diameter that is sized to catch a ball having a minimum outer diameter, and the ball 174 may dissolve over the relatively short interval of time so that the ball's outer diameter reduces below the minimum outer diameter. As another example, the ball 174 may be constructed to degrade until the ball 174 collapses, thereby allowing the ball to pass through the seat 150.

In accordance with some implementations, after the fluid barrier is formed (and confirmed via a surface pressure measurement, for example), the pressure of the tubing string 130 may be monitored until a pressure drop occurs, indicating that the fluid barrier (and ball) has been removed. Thus, some degree of waiting may occur before operations that rely on fluid communication through the toe initiator valve 150 may resume, in accordance with example implementations.

After the ball 174 degrades and the fluid barrier is removed, other operations may then be performed in the well 100, which rely on communicating fluid through the toe initiator valve 150. For example, referring to FIG. 1D, flow may be re-established in the tubing string 130, and a perforating gun assembly 180 may be pumped downhole on a wireline 181 to a region of the lateral segment 121 above the toe initiator valve 150. The perforating gun assembly 180 may then be activated to fire its perforating charges. This produces corresponding perforating jets, which perforate the wall of the tubing string 130 and form corresponding perforation tunnels 190, which extend into the surrounding formation, as depicted in FIG. 1E. In this state, the well 100 now has fluid communication with the surrounding formation to permit a well stimulation operation to be performed in the first stage above the toe initiator valve 150. For this to occur, another untethered object, such as a degradable ball 192, may be pumped downhole to once land in the object catching seat 151, as illustrated in FIG. 1F. This once again creates a fluid barrier that may be used to divert fluid uphole of the ball 192 for purposes of diverting the stimulation fluid into the stage associated with the perforation tunnels 190.

FIG. 2A depicts a partial cross-sectional view of the toe initiator valve 150, in accordance with example implementations. It is noted that FIG. 2A depicts an upper cross-sectional view of the toe initiator valve 150 about a longitudinal axis 201 of the string 130, with it being understood that the lower cross-section of the valve 150 generally mirrors the depicted upper cross-section about the longitudinal axis 201.

The toe initiator valve 150 includes a tubular housing 200, which includes the radial ports 160. The housing circumscribes a sleeve 210, which translates along the longitudinal axis 201 to open and close (the state shown in FIG. 2A) fluid communication through the port(s) 160. For the example implementation of FIG. 2A, at its uphole end 207, the toe initiator valve 150 includes an upper coupling interface, 204 to couple the housing 200 to the portion of the tubing string 130, which is uphole of the valve 150. In general, the upper coupling interface 204 may include threads 206 formed in the housing 200. As also shown in FIG. 2A, below the threads 204, the toe initiator valve 150 may, in accordance with example implementations, include the object catching seat 151, which is, for this implementation, formed by an upset, or shoulder, of the housing 200.

FIG. 2A depicts the toe initiator valve 150 in its closed state. In this state, an internal sleeve 210 of the toe initiator valve 150 blocks fluid communication through the radial port(s) 160. It is noted that the various seals between the sleeve 210 and the housing 204 are not depicted in FIG. 2A. In general, the toe initiator valve 150 may include one or multiple rupture discs 220 that are in communication with an atmospheric chamber (not shown) of the toe initiator valve 150. When the pressure inside the tubing string 130 increases above a pressure threshold established by the ruptured disc 220, the ruptured disc 220 ruptures to communicate pressure inside the tubing string 130 to the valves atmospheric chamber. This pressure imbalance, in turn, operates a piston of the sleeve 210 (not shown) to shift the sleeve 210 open to form an open state for the toe initiator valve 150, as depicted in FIG. 2B. As shown in FIG. 2B, for this state, fluid communication is open through the radial ports 160.

Referring to FIG. 3, thus, in accordance with example implementations, a technique 300 includes installing (block 304) a string having a toe initiator valve in a wellbore and opening (block 308) the toe initiator valve to establish fluid communication between the interior of the string and the region outside the string. In accordance with example implementations, the toe initiator valve may be opened by pressuring the tubing string 130 to a pressure that meets or exceeds a pressure activation threshold of the toe initiator valve, and this pressure activation threshold may be less than the maximum pressure used to pressure test the tubing string 130. Pursuant to the technique 300, fluid communication established by the open toe initiator valve is used (block 312) to pump a degradable object downhole to land in a seat of the string. A downhole operation that uses the fluid barrier resulting from the landed object may then be performed (block 316). As examples, this downhole operation may be a well stimulation operation, a pressure test, and so forth.

More specifically, in accordance with example implementations, a technique 400 that is depicted in FIG. 4 may be performed. Pursuant to the technique 400, a toe initiator valve is opened (block 402) to establish a fluid flow, and a degradable object is pumped downhole in the tubing string pursuant to block 404 to land in a seat that is associated with the toe initiator valve. Pressure testing of the string may then be performed (block 408) using the fluid barrier formed by the degradable object landing on a seat associated with the toe initiator valve. Pursuant to the technique 400, a first stage well stimulation operation using a flow through the now opened toe initiator valve may be performed, pursuant to block 412. Subsequently, a perforating gun assembly may be pumped downhole inside the tubing string to perforate the string for a second stage well stimulation operation, pursuant to block 416. The perforating guns may then be retrieved (block 420), and another degradable object may be pumped downhole in the tubing string to land in the seat associated with the toe initiator valve, pursuant to block 424. The technique 400 includes performing (block 428) the second stage well stimulation operation that uses the fluid barrier formed from the second degradable object landing in the seat.

In accordance with example implementations, one or more components of the degradable object (such as balls 174 and 192 that are described herein) may contain a material or materials, which allow at least part of the object to degrade (dissolve, structurally deteriorate, and so forth) by well fluid or another fluid, which is introduced into the tubing string passageway. As an example, the material(s) for the object may be the same or similar to the materials disclosed in the following patents, which have an assignee in common with the present application and are hereby each incorporated by reference: U.S. Pat. No. 7,775,279, entitled, “DEBRIS-FREE PERFORATING APPARATUS AND TECHNIQUE,” which issued on Aug. 17, 2010; and U.S. Patent No. 8,211,247, entitled, “DEGRADABLE COMPOSITIONS, APPARATUS COMPOSITIONS COMPRISING SAME, AND METHOD OF USE,” which issued on Jul. 3, 2012.

In this context, degradable material is a material that degrades at a significantly faster rate than other materials or components (the tubing string 130, for example) of the downhole well equipment. For example, in accordance with some implementations, dissolvable or degradable material(s) may degrade at sufficiently fast rate to allow the fluid barrier to disappear (due to the material degradation) after a relatively short period of time (a period less than one year, a period less than six months, or a period of less than ten weeks, as just a few examples). In this manner, the fluid barrier maintains its structural integrity for a sufficient time to allow the downhole operation(s) that rely on the fluid barrier to be performed, while disappearing shortly thereafter to allow other operations to proceed in the well, which rely on access through the portion of the tubing string, which contained the fluid barrier.

Other implementations are contemplated, which are within the scope of the appended claims. For example, referring to FIG. 5, in accordance with further implementations, the seat 151 may be part of a tool or component that is separate from the toe initiator valve 150. In this manner, as shown in FIG. 5, the seat 151 may be form from an inner should or upset 510, of the tubing string 130 and is disposed uphole of the toe initiator valve 150, i.e., disposed uphole of the upper threaded coupler 204 of the valve 150. As example, for these implementations, the toe initiator valve 150 may be the KickStart rupture disc valve, available from Schlumberger.

While the present techniques have been described with respect to a number of embodiments, it will be appreciated that numerous modifications and variations may be applicable therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the scope of the present techniques. 

What is claimed is:
 1. A method comprising: installing a tubing string having a toe initiator valve in a wellbore; opening the toe initiator valve to establish fluid communication between an interior of the string and a region outside of the string; using the fluid communication established by the open toe initiator valve to communicate a degradable object downhole to land in a seat of the string; and performing a downhole operation using a fluid barrier resulting from the landed object.
 2. The method of claim 1, wherein performing the downhole operation comprises pressure testing the string and using the fluid barrier to isolate the toe initiator valve from a remainder of the tubing string uphole of the fluid barrier.
 3. The method of claim 2, wherein opening the toe initiator valve comprises pressuring the tubing string to a pressure activation threshold of the toe initiator valve less than a maximum pressure used in the pressure testing.
 4. The method of claim 1, wherein performing the downhole operation comprises performing a stimulation operation and using the fluid barrier to divert fluid as part of the stimulation operation.
 5. The method of claim 1, wherein using the fluid communication established by the open toe initiator valve comprises providing a flow through a radial port of the toe initiator valve and pumping the degradable object downhole using the flow.
 6. The method of claim 1, further comprising: waiting for the object to degrade; and reestablishing fluid communication through the open toe initiator valve after the waiting.
 7. The method of claim 6, further comprising using the reestablished fluid communication through the open toe initiator valve to pump a tool downhole into the tubing string.
 8. The method of claim 6, further comprising using the reestablished fluid communication through the open toe initiator valve to communicate another degradable object downhole into the tubing string to land in the seat of the string to form another fluid barrier.
 9. A method usable with a well, comprising: opening a toe initiator valve of a tubing string; pumping a degradable object downhole to land in a seat associated with the toe initiator valve; and performing pressure testing of the string using a fluid barrier created due to the landing of the degradable object in the seat.
 10. The method of claim 9, further comprising performing a first stage well stimulation operation in the well using a flow through the open toe initiator valve.
 11. The method of claim 9, further comprising: running a perforating gun assembly downhole inside the tubing string; perforating the tubing string; retrieving the perforating gun assembly; and pumping another degradable object down to land in the seat associated with the toe initiator valve.
 12. The method of claim 11, further comprising performing a well stimulation operation using a fluid barrier formed from the landing of the other degradable object in the seat.
 13. An apparatus usable with a well, comprising: a string associated with a plurality of stages, wherein the string has a toe end and a first stage of the plurality of stages is closest to the toe end of the string; a toe initiator valve disposed in the string near the toe end of the string, the toe initiator valve comprise a radial port to communicate fluid with a region outside of the string; and a seat disposed in the string between the toe end of the string and the first stage to receive an object to form a fluid barrier between the port of the toe initiator valve and the first stage.
 14. The apparatus of claim 13, wherein the toe initiator valve is adapted to open fluid communication through the port in response to a pressure of the string exceeding 10,000 pounds per square inch (psi).
 15. The apparatus of claim 13, wherein the string comprises a well casing.
 16. The apparatus of claim 13, further comprising a degradable object to be communicated through a passageway of the string to land in the seat to form a temporary fluid barrier.
 17. An apparatus comprising: a toe initiator valve to open in response to a pressure of a tubing string in which the valve is installed exceeding a pressure threshold, the toe initiator valve comprising: a coupler to connect the toe initiator valve to a string; at least one radial port to communicate fluid in response to the toe initiator valve being open; and a seat disposed between the coupler and the port to receive an untethered object communicated through the string.
 18. The apparatus of claim 17, wherein the seat is adapted to form a fluid barrier in the string in response to the untethered object landing in the seat.
 19. The apparatus of claim 17, wherein the seat comprises a restriction in a central passageway of the toe initiator valve.
 20. The apparatus of claim 17, wherein the coupler comprises a threaded connector to connect the toe initiator valve to the string. 